Monin–Obukhov Similarity Functions for the Structure Parameters of Temperature and Humidity

Monin–Obukhov similarity functions for the structure parameters of temperature and humidity are needed to derive surface heat and water vapour fluxes from scintillometer measurements and it is often assumed that the two functions are identical in the atmospheric surface layer. Nevertheless, this assumption has not yet been verified experimentally. This study investigates the dissimilarity between the turbulent transport of sensible heat and water vapour, with a specific focus on the difference between the Monin–Obukhov similarity functions for the structure parameters. Using two datasets collected over homogeneous surfaces where the surface sources of sensible heat and water vapour are well correlated, we observe that under stable and very unstable conditions, the two functions are similar. This similarity however breaks down under weakly unstable conditions; in that regime, the absolute values of the correlations between temperature and humidity are also observed to be low, most likely due to large-scale eddies that transport unsteadiness, advection or entrainment effects from the outer layer. We analyze and demonstrate how this reduction in the correlation leads to dissimilarity between the turbulent transport of these two scalars and the corresponding Monin–Obukhov similarity functions for their structure parameters. A model to derive sensible and latent heat fluxes from structure parameters without measuring the friction velocity is tested and found to work very well under moderately to strongly unstable conditions (−z/L > 0.5). Finally, we discuss the modelling of the cross-structure parameter over wet surfaces, which is crucial for correcting water vapour effects on optical scintillometer measurements and also for obtaining surface sensible and latent heat fluxes from the two-wavelength scintillometry.     

Process-based modeling of the overflow-induced growth of erosional channels

A new process-based approach is introduced for a more efficient computation of the overflow-induced growth of an erosional channel in a non-cohesive homogeneous narrow landmass such as the breach growth in a sand-dike. The approach is easy to incorporate in a 1D/2DV morphodynamic model to compute the channel growth both vertically and laterally.     

Can the Scheldt River Estuary become hyperturbid?

Ocean Dynamics - We investigate the hypothesis by Winterwerp and Wang (Ocean Dyn 63:1279–1292, 2013) that channel deepening in the Scheldt River Estuary could lead to a large increase in...     

Evolution and structure of the Upper Rhine Graben: insights from three-dimensional thermomechanical modelling

The evolution and geometry of the Tertiary Upper Rhine Graben were controlled by a continually changing stress field and the reactivation of pre-existing crustal discontinuities. A period of WNW-ESE extension in the late Eocene and Oligocene was followed by lateral translation from the early Miocene onwards. This study utilizes 3D finite element techniques to simulate extension and lateral translation on a lithospheric scale. Brittle and creep behaviour of lithospheric rocks are represented by elastoplasticity and thermally activated power-law viscoplasticity, respectively. Contact elements allocated with cohesion and frictional coefficients are used to describe pre-existing zones of weakness in the elastic-brittle field. Our results suggest that (1) extension is accommodated along listric border faults to midcrustal depth of 15–16 km. Beneath, pure shear stretching occurs without a need for localized shear zones in lower crust and upper mantle. (2) Ductile flow at midcrustal depth across the graben accounts for the pronounced halfgraben morphology. Thereby, the shape of the border faults, their frictional coefficients, and sedimentary loads have profound effects on the rate of ductile flow across the graben. (3) Horizontal extension of 8–8.5 km and sinistral displacement across the rift of 3–4 km are needed to accommodate the observed sediment thickness.     

Gravitational orogenic collapse vs plate-boundary stresses: a numerical modelling approach to the Permo-Carboniferous evolution of Central Europe

A two-dimensional thermo-mechanical finite element model is used to study the Permo-Carboniferous evolution of Central Europe along a lithosphere-scale transect from the Variscan Internides to the undeformed foreland. The study concentrates on a quantitative evaluation of the processes controlling late-orogenic extension and destruction of the Variscides, particularly the gravitational instability of thickened crust. Modelling results suggest that gravitational forces along cannot reproduce the observed timing and amount of Permo-Carboniferous crustal thinning. Tensile plate-boundary forces are required in addition to gravitation to restore a crustal thickness of approximately 30?km in the Variscan Internides. Stresses as little as 10?MPa result in up to 28% extension and a good fit between observed data and model predictions. It is concluded that the Stephanian to Rotliegend evolution in the vicinity of the modelled traverse resulted not from gravitational forces inherited from Variscan crustal thickening, but was related to a change in orientation of the plate-boundary stresses at the end of the Westphalian.     

Importance of cross-channel bathymetry and eddy viscosity parameterisation in modelling estuarine flow

For a proper understanding of flow patterns in curved tidal channels, quantification of contributions from individual physical mechanisms is essential. We study quantitatively how such contributions are affected by cross-channel bathymetry and three alternative eddy viscosity parameterisations. Two models are presented for this purpose, both describing flow in curved but otherwise prismatic channels with an (almost) arbitrary transverse bathymetry. One is a numerical model based on the full three-dimensional shallow water equations. Special feature of this diagnostic model is that assumptions regarding the relative importance of particular physical mechanisms can be incorporated in the computations by switching corresponding terms in the model equations on or off. We also present an idealized model that provides semi-analytical approximate solutions of the shallow water equations for all three considered alternative eddy viscosity parameterisations. It forms an aid in explaining and theorising about results obtained with the numerical model. Observations regarding Chesapeake Bay serve as a reference case for the present study. We find that the relative importance of both along-channel advective forcing and transverse diffusive forcing depends on local characteristics of the cross-sectional bottom profile rather than global ones. In our reference case, tide-residual along-channel flow induced by these forcings is not small compared to the total tidal residual. Building on this observation, we present an indicative test to judge whether advective processes should be included in leading order in modelling tide-dominated estuarine flow. Furthermore, depending on the applied eddy viscosity parameterisation (uniformly or parabolically distributed over the vertical), we find qualitatively different spatial patterns for the along-channel advective forcing.     

Are water tables a subdued replica of the topography?

The water table in unconfined aquifers is often believed to be a subdued replica of the topography or land surface. However, this assumption has not been widely tested and in some cases has been found to be in error. An analysis of ground water rise in regional unconfined aquifers, using both a two-dimensional boundary element model and a one-dimensional Dupuit-Forchheimer model, reveals the conditions under which the water table does or does not closely follow the topography. A simple decision criterion is presented to estimate in advance under which conditions the water table is expected to be largely unrelated to the topography and under which conditions the topography controls the position of the water table.     

Holocene seasonal sea‐surface temperature variations in the southern Adriatic Sea inferred from a multiproxy approach

Holocene cooling events have been reconstructed for the southern Adriatic Sea (central Mediterranean) by means of analyses of organic walled dinoflagellate cysts, planktonic foraminifera, oxygen isotopes, calcareous nanoplankton, alkenones and pollen from a sediment core. Two cooling events have been detected, during which sea‐surface temperatures (SSTs) were ca. 2°C lower. Unravelling the SST signal into dominant seasonal components suggests maximum winter cooling of 2°C at around 6.0 ka, whereas the cooling at ca. 3.0 ka might be the result of a spring temperature cooling of 2–3°C. The events, lasting several hundred years, are apparently synchronous with those in the Aegean Sea, where they have been related to known cooling events from the Greenland ice‐core record. A distinct interruption in Adriatic Sea sapropel S1 is not clearly accompanied by a local drop in winter temperatures, but seems to be forced by ventilation, which probably occurred earlier in the Aegean Sea and was subsequently transmitted to the Adriatic Sea. Copyright © 2003 John Wiley & Sons, Ltd.     

Mapping East African tropical forests and woodlands — A comparison of classifiers

In mapping the forest–woodland–savannah mosaic of Budongo Forest Reserve, Uganda, four classification methods were compared, i.e. Maximum Likelihood classifier (MLC), Spectral Angle Mapper (SAM), Maximum Likelihood combined with an Expert System (MaxExpert) and Spectral Angle Mapper combined with an Expert System (SAMExpert). The combination of conventional classifiers with an Expert System proved to be an effective approach for forest mapping. This was also the first time that the SAMExpert had been used in the mapping of tropical forests. SAMExpert not only maps with high accuracy, but is also fast and easy to use, making it attractive for use in less developed countries. Another advantage is that it can be executed on a standard PC set up for image processing.Combining the conventional classifiers (MLC and SAM) with the Expert System significantly improved the classification accuracy. The highest overall accuracy (94.6%) was obtained with SAMExpert. The MaxExpert approach yielded a map with an accuracy of 85.2%, which was also significantly higher than that obtained using the conventional MLC approach.The SAMExpert classifier accurately mapped individual classes. Of the four classes of woodland mapped, the Open Woodland (with Terminalia) and Wooded Grassland classes were more accurately mapped using SAMExpert. The Open Woodland had been previously identified by ecologists, but had never been mapped.